There is a need to rapidly assay or screen potential drug candidates. Drug discovery is a long, multiple step process involving the identification of specific disease targets, development of assays based on a specific target, validation of the assays, and optimization and automation of the assay to achieve screening of a large number of candidates. After high throughput screening of compound libraries using various assays, hit validation and hit compound optimization procedures are employed. Performing a screen on many thousands of compounds thus requires parallel processing of many compounds and assay component reagents. In addition, to find lead compounds for drug discovery programs, large numbers of compounds are often screened for their activity as enzyme inhibitors or receptor agonists/antagonists. Large libraries of compounds are needed for such screening. As a result of developments in this field, it is now possible to simultaneously produce combinatorial libraries containing hundreds of thousands of small molecules for screening. With such libraries on hand there is an ever increasing need to rapidly screen these new drug candidates.
One common approach to drug discovery involves presenting macromolecules implicated in causing a disease (disease targets) in bioassays in which potential drug candidates are tested for therapeutic activity. Such molecules may be receptors, enzymes, transcription factors, co-factors, DNA, RNA, growth promoters, cell-death inducers, or non-enzymatic proteins and peptides. Another approach involves presenting whole cells or organisms that are representative of the causative agent of the disease. Such agents include bacteria and tumor cell lines, for example. Thus, there is a need to be able to screen the effects of various drug candidates on assorted cells and cell lines.
Within the general drug discovery strategies, several sub-strategies have been developed. One spatially-addressable strategy that has emerged involves the generation of peptide libraries on immobilized pins that fit the dimensions of a standard 96-well microtiter plate. See PCT Patent Publication Nos. 91/17271 and 91/19818, each of which is incorporated herein by reference. This method has been used to identify peptides that mimic discontinuous epitopes as described in Geysen et al., “Screening Chemically Synthesized Peptide Libraries for Biologically Relevant Molecules,” Bioorg Med Chem. Lett. 3: 397-404 (1993), and to generate benzodiazepine libraries as described in U.S. Pat. No. 5,288,514 and Bunin et al., “The Combinatorial Synthesis and Chemical and Biological Evaluation of a 1,4-Benzodiazepine Library,” Proc. Natl. Acad. Sci. 91: 4708-4712 (1994). The structures of the individual library members can be determined from the pin location in the microtiter plate and the sequence of reaction steps (called a “synthesis histogram”) performed during the synthesis.
Common therapeutic targets for high throughput screening (“HTS”) are enzymes, cell surface receptors, nuclear receptors, ion channels, signal transduction proteins, cell surface glycoproteins and proteoglycans. Compounds that interact with these targets are usually identified using in vitro biochemical assays.
These assays and other conventional and HTS assays, such as DNA analysis, gene expression profiling, mapping for single nucleotide polymorphisms (SNP's), and enzyme linked immunosorbent assay (ELISA) and others rely on arraying of biomolecules.
In a recent development, the techniques of photolithography, chemistry and biology have been combined to array large collections of biocompounds on the surface of a substrate. See U.S. Pat. No. 5,143,854 and PCT patent publication Nos. 90/15070 and 92/10092, each of which is incorporated herein by reference.
“Genechip” technology as well as photolithography to generate patterns of target oligonucleotides (See U.S. Pat. No. 5,599,695) have revolutionized the ways assays are performed. Robotic spotting systems have been developed to “print” arrays of nucleic acids and other materials on surfaces for assay development. Unfortunately, both photolithography and robotic array systems require expensive equipment. In addition, processing conditions required in photolithography are often incompatible with many biochemical and biological materials. Robotic spotting systems work well when used with homogeneous fluids, but they do not work efficiently when attempting to pattern cells directly. Although silkscreen printing and ink-jet printing have recently shown much promise in the generation of biological arrays, they suffer from their inability to generate patterns having fine resolution. Obtaining sub-100 μm resolution is difficult with these techniques.
Conventional methods for assessing the effects of various agents or physiological activities on biological materials utilize standard microtiter plates. See, e.g., U.S. Pat. No. 6,083,763. Unfortunately, microtiter plates are not flat, but comprise portions which protrude substantially perpendicular from the surface on which the items to be detected are found. These perpendicular protrusions are fabricated from plastic, for example, and form the wells that separate different fluids and immobilized entities, such as cells, organic molecules, and biomolecules. These perpendicular protrusions are not removable, and they hinder the use of many detection systems, such as, microscopes including bright field, phase contrast, confocal, and epi-illuminated fluorescence microscopes, MALDI (Matrix Assisted LASER Desorbtion Ionization) mass spectrometers, surface plasmon resonance (SPR) sensors, and flat bed scanners including confocal flat bed scanners. For example, perpendicular protrusions can interfere with microscope objectives, prohibiting the user from bringing the microscope lens closer to the sample than above the height of the perpendicular protrusions.
The perpendicular protrusions in microtiter plates and the like also hinder the exposure of material immobilized in all or a plurality of wells to the same fluids bearing reagents and the like, and also hinder washing between steps. Fluids must generally be pipetted into individual wells because fluids applied to the surface of a microtiter plate will generally not enter the wells, but rather will remain on the surface, due to surface tension; this will also be the case for microtiter plates immersed in fluids. Fluids that have been pipetted into wells are difficult to remove from wells of the microtiter plates without introducing possible contamination during the fluid removal process.
There are currently efforts to overcome the aforementioned shortcomings of arrays by immobilizing materials of interest at defined locations on a flat surface. However, these efforts arc limited by the difficulty of creating discrete spots of material, particularly where the material differs between spots, without physical mechanisms for separation of the spots while they are being formed. Thus, although immobilized spots remain discrete in the absence of physical barriers between them, without physical barriers, it is difficult to maintain separation between spots as they are being immobilized.
Thus, there is a need for a device that allows for the immobilization of materials in discrete areas, or spots, in a spatially defined array and which also (1) facilitates reading the results of assays and the like conducted on the device in systems which require or are better functioning when reading flat surfaces and (2) allows for easy and efficient exposure of multiple spots to a fluid or fluids.